Abstract
A model is proposed on the exchange coupled singlet-ground-state magnet around the level crossing field. The model starts from the energy level of the isolated single spin in paramagnetic spin system. The effect of the exchange coupling is taken into account by modifying the single ion level scheme to the broadened continuum band. On the basis of this model is discussed a mechanism of the nuclear spin-lattice relaxation inherent in the level crossing, due to the direct process between the nuclear spin and the exchange coupled electron spin system. A drastic increase of the nuclear spin-lattice relaxation rate of 133Cs in CsFeCl3 observed at 4.2 K around the level crossing field at 7.5 T is well explained by this model. The magnetization as a function of external magnetic field is also discussed and compared with the result of CsFeCl3 up to 12 T.
